1. Field of the Invention
This invention relates to a semiconductor device, its manufacturing method and a substrate for manufacturing a semiconductor device, and more particularly to a semiconductor device, like a semiconductor laser, having cavity edges made by cleavage, its manufacturing method, and a substrate, like a wafer, used for manufacturing such a semiconductor device.
2. Description of the Related Art
Nitride III-V compound semiconductors, such as GaN, AlGaN and GaInN, made of a group III element, such as gallium (Ga), aluminum (Al) and indium (In), and nitrogen as a group V element, are direct-transitional semiconductors, and they have larger band gaps than those of semiconductors such as AlGaInAs and AlGaInP used in currently available semiconductor lasers. Therefore, they are expected to be widely applicable as light sources of high-integrated, high-density optical disc reproducing apparatuses and optical elements for full-color display devices, in form of short-wavelength semiconductor lasers for emission wavelengths in the band of 400 nm, light emitting diodes (LED) and other semiconductor light emitting devices capable of emitting ultraviolet to green light. Moreover, these nitride III-V compound semiconductors exhibit large saturation electron velocities under a high electric field, and are remarked as materials of electron-mobility devices such as field-effect transistors (FET) for high powers and high frequencies.
Semiconductor lasers, light emitting diodes and FETs using these nitride III-V compound semiconductors are made by epitaxially growing nitride III-V compound semiconductors on a substrate such as sapphire (A1.sub.2 O.sub.3) substrate, for example.
In semiconductor lasers, in general, cavity edges must be made. In AlGaInAs, AlGaInP or InP semiconductor lasers, substrates and semiconductor layers grown thereon are cleavable, and cleavable surfaces are normally used as cavity edges of the semiconductor lasers.
In case of nitride III-V compound semiconductors, however, it is usually difficult to make stable cleavable surfaces because their crystallographic structures are hexagonal system wurtzite structures. Moreover, since these semiconductor lasers using nitride III-V compound semiconductors are usually made by growing nitride III-V compound semiconductors on sapphire substrates which are not cleavable, it has been difficult to fabricate semiconductor lasers using cleavable surfaces as cavity edges.
Japanese Patent Laid-Open Publications Nos. hei 8-222807 and hei 9-172223, for example, disclose methods for manufacturing GaN semiconductor lasers in which cavity edges are made by cleaving a sapphire substrate and III-V compound semiconductor layers stacked thereon.
More specifically, as shown in FIG. 1, these conventional methods for manufacturing GaN semiconductor lasers sequentially grow a GaN buffer layer 102, n-type GaN contact layer 103, n-type AlGaN cladding layer 104, active layer 105 of a GaN/GaInN multiquantum well structure, p-type AlGaN cladding layer 106 and p-type GaN contact layer 107 on a c-plane sapphire substrate 101 by metal organic chemical vapor deposition (MOCVD).
Next made on the p-type GaN contact layer 107 is a resist pattern (not shown) in form of a predetermined stripe. Using the resist pattern as a mask, reactive ion etching (RIE) is conducted to selectively remove upper layers including an upper part of the n-type GaN contact layer 103. As a result, the upper-lying part of the n-type GaN contact layer 103, n-type AlGaN cladding layer 104, active layer 105, p-type AlGaN cladding layer 106 and p-type GaN contact layer 107 are patterned into a predetermined mesa structure extending in a direction. Numeral 108 denotes the mesa portion.
After the resist pattern is removed, the p-side electrode (not shown) is made on the p-type GaN contact layer 107, and the n-side electrode (not shown) is made on the n-type GaN contact layer 103 in the partly removed region.
After that, the wafer-shaped sapphire substrate 101 having formed the laser structure is lapped from its bottom surface to adjust the thickness of the sapphire substrate 101 to approximately 150 .mu.m. Then, in locations of the bottom surface of the sapphire substrate 101 for making cavity edges, which may be locations corresponding to (11-10)-oriented surfaces, straight cleavage-assist grooves 109 are made to extend in parallel to the (11-20)-oriented surfaces. Thus, in the direction parallel to the lengthwise direction of the mesa portion 108, that is, in the cavity direction, a plurality of cleavage-assist grooves 109 are made periodically in intervals approximately the same as the cavity length of the GaN semiconductor lasers to be finally made.
The sapphire substrate 101 is next cleaved into bars together with the semiconductor layers thereon along the cleavage-assist grooves 109 to make opposite cavity edges, and the bars are divided into chips. As a result, the intended GaN semiconductor laser is completed.
The conventional method for manufacturing a GaN semiconductor laser can make cavity edges of cleavable surfaces (quasi-cleavable surfaces) more excellent in optical characteristics than those of cavity edges made by etching semiconductor layers forming the laser structure.
However, the conventional method for manufacturing a GaN semiconductor laser involves the following problems.
In most semiconductor lasers, the optical cavity length is designed to 1 mm or less, more particularly, in the range of 0.2 to 0.7 mm, approximately. However, in order to minimize the optical cavity length to these values, the thickness of the sapphire substrate 101 must be reduced by lapping. For example, unless the thickness of the sapphire substrate 101 is 150 .mu.m or less, the sapphire substrate 101 and overlying semiconductor layers do not readily divide along the cleavage-assist groove 109, and it was difficult to make cavity edges acceptable in optical evenness at the desired position.
Moreover, since the sapphire substrate 101 is chemically stable, it is difficult to selectively etch semiconductor layers made of nitride III-V compound semiconductors layers, or insulating films such as SiO.sub.2 film and SiN film, which are made on the sapphire substrate. It is therefore difficult to chemically process the sapphire substrate 101 alone while protecting a part of the crystal growth surface and bottom surface. Therefore, in order to make cleavage-assist grooves 109 in the sapphire substrate 101, dicing, scribing or other mechanical processing was necessary, and there arose problems in pattern accuracy and micro processing of the cleavage-assist grooves 109.
Since the sapphire substrate 101 decreases in strength with a reduction in thickness, if the sapphire substrate 101 is made thinner, then the sapphire substrate 101 is apt to crack to its surface or break while the cleavage-assist grooves 109 are made on the bottom surface of the sapphire substrate 101 by using a dicer or scriber, for example. In this case also, it was impossible to make acceptable cavity edges. For the purpose of preventing cracks of breakage of the sapphire substrate 101, there arose the need for minimizing varieties in thickness of the sapphire substrate 101 by controlling the thickness of sapphire substrate 101 after lapping, and the thickness of the sapphire substrate 101 in locations of the cleavage-assist grooves 109 after being made. Furthermore, as the sapphire substrate 101 was made thinner and thinner, warp of the substrate became too large to handle the substrate due to thermal stress caused by a difference in thermal expansion coefficient between the sapphire substrate 101 and semiconductor layers grown thereon, and/or damages by lapping, or the like.